This invention relates generally to improvements in the manufacture of circuit boards. More particularly, this invention relates to apparatus and method for use primarily with so-called multiple layer circuit boards wherein selected holes in the board may be filled and sealed quickly and easily with an epoxy resin or the like to prevent degradation of conductive material lining the holes by exposure to a corrosive atmosphere, moisture, or the like.
Circuit boards in general are known in the art and typically comprise a substrate formed from an insulating material such as a sheet of glass epoxy or the like having prescribed patterns of electrically conductive material, such as copper, imprinted on the opposite sides thereof. These conductive patterns can be electrically communicated with each other by one or more holes drilled through the board wherein these holes have their linings plated with additional electrically conductive material to complete the electrical path from one side of the board to the other. In addition, some of these holes define convenient mounting stations for receiving mounting prongs of electronic elements, such as transistors, resistors, capacitors, solid state chips, and the like, which operate in conjunction with the conductive patterns to define the desired electrical circuit.
According to modern circuit board production techniques, the circuit board sometimes includes a plurality of relatively thin glass epoxy substrate layers separated from each other by a plurality of specifically configured conductive patterns to form a so-called multilayer circuit board. In a circuit board of this type, a significant number of holes are formed through the board and lined with conductive material for providing the desired electrical path between the various conductive patterns. Many of these holes are not required for receiving mounting prongs of electronic elements and thus it is desirable to fill and/or seal these holes with an appropriate substance which will prevent the conductive material lining the holes from degrading or being attacked by corrosive substances. In addition, filling these holes can result in an improved overall structural strength for the board.
In the prior art, this filling and sealing process has been accomplished by spreading an epoxy resin or the like over an upper surface of the board and by forcing the resin with a squeegee or similar manual tool into the open holes in the board. This manual process, however, is inconsistent with modern circuit board production techniques desiring relatively high production rates. Moreover, the epoxy resin is spread over substantial surfaces areas of the board and into some of the holes where it is not desired. This resin must be removed after curing by extensive and time-consuming sanding of the board surface and by redrilling of the holes to be utilized as mounting stations for electronic elements.
The present invention overcomes the problems and disadvantages of the prior art by providing a novel apparatus and method for filling and sealing selected holes in a multilayer circuit board with a filling material, such as an epoxy resin, wherein the resin is prevented from contacting undesired portions of the board, and in a manner consistent with modern techniques for high production rates.